1. Field of the Invention
This invention relates to a cracking furnace and more particularly it relates to a tubular furnace for thermal cracking of hydrocarbons and like organic feedstocks, especially petroleum hydrocarbons into ethylene, propylene, butadiene, etc.
2. Description of the Related Art
FIG. 4 of the accompanying drawings shows a cross-sectional view of a cracking furnace according to the prior art, FIG. 5 shows a cross-sectional view in the direction of arrow mark along the line V--V of FIG. 4, and FIG. 6 shows a cross-sectional view of a quenching heat exchanger connected by the medium of a header to the exits of the reaction tubes of FIG. 4.
The above apparatus comprises a plurality of reaction tubes 7 provided in a row along the longitudinal direction and at the central part of combustion chamber 1, the reaction tube forming a meander having a bend 8; inlet 21 of an organic feedstock provided at one end of the reaction tube 7; exit 22 of the reaction tube 7 provided at the other end thereof; quenching heat-exchanger 10 united by the medium of header 9 (FIG. 6) to the exit 22 of the reaction tube; reaction tube-suspending fittings 16 suspended from the ceiling of combustion chamber 1 and connected to counterweight 17; convection heat transfer tubes 11 provided at convection heat transfer part 6 communicated to the upper part of the combustion chamber 1 by the medium of connecting duct 5; hearth burners 20 provided vertically at the bottom part of the combustion chamber 1; and wall burners 19 provided on the wall surface of the combustion chamber 1.
In such an apparatus, an organic feedstock is usually mixed with steam; preheated at convection heat transfer part 6; thereafter introduced via reaction inlets into reaction tubes 7; heated by radiation from hearth burners 20 and wall burners 19 to cause pyrolytic reaction; introduced via header 9 into quenching heat exchanger 10; and quenched so that cracked products are obtained without causing excess pyrolytic reaction or polymerization reaction.
In the cracking furnace, when ethylene is produced on the base of 30,000 t/year from naphtha (specific gravity: 0.70), it is necessary to make the length of the cracking furnace around 15 m, and in the case of 50,000 ton/year, 20 m or more of the length is required; thus the site area per ton of naphtha becomes broader. Further, there occurs a drawback that the heat loss from the furnace wall increases. Further, when combustion gas 23 in combustion chamber 1 moves toward convection heat transfer part 6, it is necessary for the gas to traverse reaction tube-suspending fittings 16 so that they are liable to be damaged due to the high temperature gas. Further, since connecting duct 5 connecting combustion chamber 1 to convection heat transfer tube 11 is short when combustion gas having non-uniform temperatures is not yet sufficiently mixed, the gas is introduced into convection heat transfer tubes 11 which usually consist of 4 to 12 passes so that the convection heat transfer tubes 11 corresponding to respective passes are non-uniformly heated, and as a result, the temperatures of the respective passes at the exits of the convection heat transfer tubes are different. Some large difference can be in the amounts to 20 to 50.degree. C.
Ideal reaction tubes have a small pressure-loss; can rise up to a definite temperature within a definite retention time and carry out the maximum treatment of the feedstock per one pass; and can provide as short as possible, a connection length of the exits of the reaction tubes to the inlet of the quenching heat-exchanger by way of header 9. Further, it is desirable from an economical point of view to use a small number of quenching heat exchangers each having a large capacity.
According to the above cracking furnace, however, in order to reduce the number of quenching heat exchangers 10 that are installed, reaction tubes having a complicated bend or a bend of a small bending radius, Y-shaped collecting tubes piece, etc. as shown by the configurations of various reaction tubes of FIG. 8 and FIG. 9 are used. Thus, the smooth flow of the fluid in the furnace is hindered, which cause an increase in the pressure loss and coking. Further, when the configuration of reaction tubes is complicated, there is a drawback that the reaction tubes and bending parts are liable to be damaged due to the occurrence of extraordinary thermal stress under high temperature condition (750.degree. to 1,100.degree. C.). The configuration of the reaction tube of FIG. 5 is very simple, but the quantity of feedstock per one pass is so small that there is a drawback that it is necessary to use a large number of quenching heat exchangers each having a small capacity. For example, in the case where ethylene is produced on the base of 30,000 ton/year from naphtha, it is necessary to provide 16 or more quenching heat exchangers.